Powder flame spraying apparatus, and particularly equipment designed to apply a layer of hardfacing to metal, dates back many decades. Designers have evolved many variations of high temperature torch equipment for heating a metal surface to a semi-molten condition and feeding abrasion-resistant metal on to such a surface to form a layer of hardfacing. Prior proposals most pertinent to the present invention are disclosed in United States patents to Bleakley Nos. 2,233,304; Wett 2,671,689; Kough 2,787,497; Lamb 2,957,630; Shepherd et al 3,111,267; Cape 3,281,078; Cape 3,352,492; Hawk Sr. 3,404,838; Hawk Sr. 3,415,450; Hawk Sr. 3,436,019; Broderick et al 3,620,454; and Broderick et al 3,995,811.
Each of these patents show means for delivering powder from a reservoir by gravity past a manually operated valve for regulating powder flow. In some of these designs the powder flows into a pressurized gas which discharges the powder and gas mixture into the fuel mixture en route to the burner nozzle or nozzles. Such arrangements are shown in Bleakley Nos. 2,233,304; Wett 2,671, 689 and Hawk Sr. 3,415,450. Others such as Cough 2,787,497; Cape 3,352,492; Broderick et al 3,620,454 and 3,995,811 deliver the powder by gravity directly into the flame jets themselves. However, the remainder of the first listed patents disclose torches in which the powder flows by gravity without a conveying gas stream directly into the fuel mixture en route to the torch flame.
Each of these prior powder delivery systems is subject to serious shortcomings and disadvantages because of the inadequate and inefficient means provided for controlling the quantity or rate of powder flow, the flow being dependent on the operator's manipulation of the flow control means and his guesstimate of the amount of powder flowing at any given time as he manually holds the flow control valve open against a valve closing spring. Additionally the rate of powder flow when mixed with fuel flowing to the torch nozzle varies with fuel flow as adjusted by the operator thereby requiring further guesstimate of the powder valve adjustment.
Another serious shortcoming of prior equipment is the lack of any means for pre-heating the metal to be coated in advance of and prior to the delivery of the powder to the preheated area. In consequence, portions of the powder impinge upon areas of the metal not heated to fusion temperature with resulting loss of very expensive powder.
It is well known that all oxy-acetylene torches, or the like, are typically subject to "backfire" from various causes as, for example, reduced fuel line pressure, accidental obstruction of the nozzle outlet, or operating the torch too close to the workpiece. In consequence, the flame propagation recedes back into the fuel passage leading to the nozzle. This usually extinguishes the torch, accompanied by a disturbing sharp report.
If the torch is being used to feed powder in a hardfacing operation wherein the powder flows by gravity into the nozzle mixing chamber, the backfire is not only accompanied by a disturbing report but can cause an explosion extremely hazardous to both the equipment and the operator. This is because the backfire flame can extend into the powder reservoir causing serious increases in pressure, a shut down in operations, the loss of valuable powder, the repair or replacement of damaged components and severe injuries to the operator.